(a) Field of the Invention
This invention relates to novel compositions and an assembly process for the manufacture of liquid crystal displays.
(b) Background
Liquid crystal displays (LCDs) have been the predominate display choice for decades for many applications including watches, radios, PDAs, portable computers, tablet computers and projection TVs because of their flatness, light weight, high definition, low driving voltages and low power consumption. However, even with many improvements in materials and assembly processes, large-size LCDs are still very difficult and time-consuming to manufacture.
The basic components of a typical LCD consist of a thin layer of liquid crystals, two plates each coated with an alignment layer and a pair of polarizers. In a typical LCD assembly process, the first step involves the preparation of the electrode plates. For active matrix color displays, the deposition and patterning of color filter elements are often carried out on the top electrode plate and the formation of thin film transistors and metal interconnect lines is carried out on the bottom plate. For passive color displays, transparent electrodes are patterned on both facing plates in perpendicular arrays and color filters on the viewing plate provide the full color display. After the electrode plates are prepared, alignment layers are then coated on the electrode plates and rubbed or buffed carefully to control the orientation and pretilt angle of the liquid crystals. The top and the bottom plates are then joined together using an edge seal adhesive with small hole(s) open for injection of a liquid crystal (LC) composition by, for example, a vacuum filling process, into the gap between the two plates. Spacers such as mono-dispersed particles, fibers or relief patterns prepared by lithography are used to precisely control the cell gap. Finally, the holes are filled with a sealant and the display cell is laminated with polarizer films and optionally a retardation film, compensation film or light management film, connected with driver circuitry, and assembled with a backlight unit and a frame to complete the display assembly.
In the conventional LC injection process, the space created between the two plates is evacuated and the LC composition is introduced into the cell through the hole(s) left in the edge sealant. This step results in significant waste of the LC composition and is typically a very slow process particularly for LCDs having a narrow cell gap. As the display size increases, the time required for the LC filling using the conventional process increases dramatically. The requirement of a narrower cell gap, in-plane switching (IPS) for a high speed or wide viewing angle renders the LC filling step even slower. For example, 4 to 6 hours of the filling and sealing time is typically required for a 15″ LCD and up to 20 hours has been reported for a 22″ LCD. Review of the traditional LCD assembling processes can be found in “Liquid Crystal Flat Panel Displays: Manufacturing Sciences & Technology” by W. C. O'Mara (1993); “Flat Panel Display Handbook”, Display Industry Technology Review, 2nd ed., (2000) by Stanford Resource, Inc. and “Flat Panel Display 2002 Yearbook” by Nikkei Microdevices.
Filling the LC composition before assembling the two electrode plates (i.e., the “LC dropping method”) has been disclosed to shorten the LC filling time [see S. Yanada et al in SID 01 Digest, pp. 1350 (2001); H. Kamiya et al, in SID 01 Digest, pp. 1354 (2001) and references cited therein]. The LC dropping process includes (1) applying a UV edge sealant on an electrode plate, (2) dropping the liquid crystals onto the surface, (3) assembling display cell by adhering a second substrate under vacuum to the first electrode plate via the edge sealant, and (4) hardening the sealant by, for example, UV. The process reduces the LC filling time and also reduces waste of the expensive LC material. However, the process is not straight-forward for the production of large display panels. First of all, a vacuuming step is needed to assemble the display cell. Secondly, a precise volume of the LC composition must be dripped onto the plate. Use of excess of the liquid crystal composition may result in defects or poor adhesion due to undesirable contamination of the sealant by the LC composition. On the other hand, insufficient liquid crystal composition tends to result in undesirable air pockets trapped in the display cell. As the panel becomes larger and the glass substrate becomes thinner, the electrode plate may be bended by vacuum and deviation of the LC volume in the cell may occur. This is particularly true when soft or mobile spacers are used to define the cell gap. Thirdly, non-uniform hardening of the edge sealant may occur due to the presence of opaque patterns in most TFT (thin film transistor) LCDs. Finally, this process is difficult to scale up to roll-to-roll manufacturing.
Roll-to-roll filling and sealing processes have been disclosed in a copending patent application, Ser. No. 09/759,212 (corresponding to 2002-0126249), for dispersed type LCDs. The content of this co-pending application is incorporated herein by reference in its entirety. However, the two continuous filling/top-sealing processes disclosed therein are not useful for most LCDs if an alignment layer is needed to control the orientation and pretilt angle of the liquid crystals.